1. Field of the Invention
The present invention relates to an injection molding system and, more particularly, to: the energy management of a hybrid injection molding system that comprises (i) an electrically driven prime mover of a hydraulic pumping assembly and (ii) a plurality of hydraulic and/or electric driven actuators (such as parts handling robots, extruders, injection units, mold stroke, and clamping units, and the like); and to the energy management of an all-electric injection molding machine system that comprises a plurality of electric driven actuators (such as parts handling robots, extruders, injection units, mold stroke and clamping units, and the like).
2. Description of Related Art
Actuators and of conventional injection molding machines typically use a hydraulic power source. It is well known that hydraulic systems are not energy efficient. This is primarily due to its inherent volumetric losses and torque losses. Volumetric losses include laminar leakage losses, turbulent leakage losses, and losses due to fluid compressibility. Torque losses include losses due to fluid viscosity and mechanical friction.
Electro-hydraulic drives and controls are based on two main operating principles: (i) valve control, by changing the resistance to flow in the conductive part, and (ii) pump control, by changing the volume flow in the generative part of a hydraulic power system. Valve-controlled drives regulate the energy flow by dissipating the excess energy. This is not energy efficient but can achieve quicker responses and better controllability, which are required by a high performance injection molding machine.
In a pump-controlled drive, the energy flow is regulated in accordance with the demand. It accomplishes control by changing the swivel angle of a variable displacement pump, or the speed of a fixed displacement pump, or the speed and the swivel angle of a variable displacement pump. Both speed dependent losses and noise emissions can be considerably reduced by applying speed controlled fixed displacement pumps. The pump/motor efficiency depends on variables such as displacement, pressure difference, and rotational speed. When a variable displacement pump is operating at small displacements, both the laminar leakage and torque losses are relatively large, thus reducing the pump/motor efficiency. In actuation systems using adjustable speed prime mover driving variable displacement pumps, the hydraulic states (namely volume, flow, and pressure) can be controlled as usual through the adjustment of the swivel angle of the pump together with the adjustment of the drive speed. This two-degree-of-freedom regulation helps to improve the pump/motor efficiency at small displacements while minimizing the supply of energy to an injection molding machine.
More recently, cost reduction and improved reliability of power electronics have made actuators driven by servo electric motors more practical for injection molding machines. A Voltage Source Inverter (VSI) is one such drive. A VSI drive includes a converter converting the AC power to DC source, a voltage controlled DC link smoothing the rectified DC voltage with a capacitor and an optional inductor, and an inverter with control generator supplying regulated power to control a motor. Injection molding machines using solely electric motors as propulsion means are commonly called all-electric machines. Injection molding machines using both hydraulics and servo electric motors as propulsion means are commonly called hybrid injection molding machines.
In a conventional hydraulic injection molding machine, the number of poles and the supply frequency of its prime mover (often an Induction Motor (IM)) are fixed. As a result, the hydraulic pumps are driven at a constant speed, even though the demand varies considerably during the cycle. When flow demand is reduced, the excess flow rate is bypassed to the accumulator tank by a relief valve. One way of meeting the varying demands is to fit an Adjustable Speed Drive (ASD) to the motor. An ASD allows the speed of an AC motor to be varied and the pump output can therefore be matched to the variable demand. The benefits of the ASD are: reduction in energy costs by matching the output to the need, reduction in noise by running the motor and pump at lower speeds, reduction in throttle and bypass losses, and a reduction in the oil cooling cost.
One control strategy is based on matching the demands of each machine operation phase, such as those disclosed in U.S. Pat. No. 5,052,909. The machine controller controls the pump motor to change the speed at each point of the process. The more closely the controller commands the right speed to match the actual requirements at each point of the process, the more energy is saved. However, there are drawbacks to this approach. When the drive accelerates and decelerates slowly, the speed to produce the desired flow must be commanded well before the flow is needed in order to deliver the right amount of flow when it is demanded. The extra flow produced during the time of such acceleration does not produce useful work, since the machine is not yet ready to receive the flow. The same happens on deceleration. Deceleration to a new lower flow can only commence when the higher flow is not needed. If the drive cannot slow down very quickly, the extra flow is also wasted. Even if the motor can accelerate and decelerate rapidly, for a prime mover with high inertia, significant kinetic energy that is stored during acceleration is often wasted during deceleration. Therefore, merely using an ASD to change the speed of the pump motor for matching the variable demands is not the most effective way to improve the energy efficiency.
During deceleration, the electric motor acts as a generator and energy is fed back to the drive, which if not managed, may raise the voltage of a DC link to an unacceptable level that renders the drive inoperative. Known systems use braking resistors and chopping circuits to dissipate regenerative energy. This energy is dissipated in the form of heat and cannot be reused. In other known systems, regenerative units in the form of Active Front End (AFE) are used to invert the regenerative energy to AC power and feed this back to the supply system. AFE provides bi-directional energy exchange between the supply and the inverter, and it generates lower harmonics than a diode bridge rectifier. However, it requires that the DC bus voltage to be greater than the peak of the AC input voltage; otherwise the sinusoidal wave shape of the output current cannot be maintained.
To be effective, ASD with AFE operates generally at a higher DC link voltage than ASD with diode bridge rectifier; it results in higher rates of voltage change at the motor terminals. AFE adds an extra set of Insulated Gate Bipolar Transistors (IGBT) inverters to the ASD and it has twice the cost of diode bridge rectifier; and it causes the ASD to generate a net increase of Electro-Magnetic Interference (EMI), unless filtering measures such as adding an isolation transformer and/or inductors are taken. From a machine manufacturer's perspective, it is advantageous to supply customers with cost effective equipment. Together with their enabling accessories, solutions using either braking resistors or AFE all add cost to the system and may not be justified or desirable for all applications. It is apparent that management of these regenerative energies for the safety of the system and capturing them for reuse are the challenges which both all-electric and hybrid injection molding machines have to face. Therefore, more economical and effective means to achieve higher energy efficiency for an injection molding system are needed.
The power demand during an injection molding cycle is not constant and the average power requirement is substantially less than the peak demands. The machine designer often takes advantage of such a power requirement pattern by installing an energy storage device, such as an accumulator, to reduce the installed power. When properly sized, the energy storage devices are charged with energy, during the low power demand operations, of the machine cycle. When high power demand is required, the energy storage devices supply energy in addition to the installed power devices. In this way, the installed power can be reduced. Several known systems use electrical energy storage devices with charging and discharging means connected to the DC link of an electric drive to accumulate the regenerated energy for future use.
Electro-chemical batteries and electrolytic capacitors are the state of the art electrical energy storage devices commonly used for such a purpose. Batteries that store energy in electro-chemical cells are subject to several limitations. One such limitation is the service life, which is the number of charge and discharge cycles possible for a given cell. Another limitation is the depth of discharge, which is that fraction of the stored energy that can be withdrawn. Further, the ambient temperature and the proper charging current must be monitored and kept within limits. The depleted materials may be hazardous and require additional cost for their disposal.
Electrolytic capacitors used as energy storage devices also have several drawbacks in the areas of size, weight, cost, and reliability. Specifically, reliability is a major concern. Due to continuous out-gassing, the electrolytic capacitors deteriorate gradually over time, and this is normally the prime factor of degradation of the system reliability, and consequently they limit the service life of a drive system. In injection molding cycles where cycle time is short and peak demands are high (such as the molding of thin wall parts), a large energy storage device is needed. However, the service life of large electrolytic capacitors and batteries is short and is limited by frequent withdrawal of peak energy. Therefore, electrolytic capacitors are not preferred for use in injection molding applications.
A hydraulic accumulator, as an energy storage device, has the ability to accept high frequencies and high rates of charging and discharging, both of which are not available from electro-chemical batteries and electrolytic capacitors. Typical average efficiency of a hydraulic accumulator is 95–97%. Use of hydraulic accumulator, as energy storage device in injection molding machine, is well known. However, applying it to capture regenerated electrical energy in hybrid injection molding machines is novel. This is partly due to the lack of simple conversion means and transfer means to convert and divert the regenerated electrical energy to a hydraulic accumulator. Even should such means exist, there is no control means in any injection molding system to manage the exchange and regulation of energies in a safe, effective and efficient manner. Therefore, new solutions are needed. This invention discloses new methods and means to solve these problems.
U.S. Patent Publication No. 2003/0089557A1 to Ellinger describes a system for operating elevators that uses a super capacitor to store energy. The application does not teach a method of power balancing at the common DC link nor how the disclosure may relate to injection molding equipment.
U.S. Pat. No. 6,611,126 to Mizuno describes the use of an electrical charger, capable of storing and supplying electrical power to a motor, connecting to the same DC link as the inverter that controls the same motor. The patent does not discuss the use of hydraulic accumulator to store regenerated energy during the deceleration of electric axes. The patent provides no teaching regarding a method of power balancing at the common DC link.
U.S. Pat. No. 6,647,719 to Truninger describes an electrical power control system for machines. An electric motor drives a pump that supplies pressurized oil to a hydraulic circuit that includes actuators and accumulators such that the system acts like an oscillator. The patent provides no teaching regarding a method of power balancing at the common DC link.
U.S. Pat. No. 6,379,119 to Truninger describes an open hydraulic circuit with a vertical load referenced that is supplied by a pump driven by an adjustable speed electric motor. The patent provides no teaching regarding a method of power balancing at the common DC link.
U.S. Pat. No. 6,333,611 to Shibuya describes the use of an electricity accumulation means to accumulate electrical energy regenerated from a motor to build up an electricity accumulation voltage higher than a drive voltage of the same motor, and to supply such electrical energy accumulated to the same motor during an acceleration period of the same motor. The patent does not discuss the management of a common DC link, supplying a plurality of electric servo axes, and it does not discuss the use of hydraulic accumulator to store regenerated energy during the deceleration of electric axes.
U.S. Pat. No. 6,299,427 to Bulgrin describes controls for adjustable speed motor driven pumps. According to the patent disclosure, each motor/pump, driving an axis of an injection molding machine, is independently controlled to match cycle requirements. The patent does not discuss the advantages of using hydraulic energy storage means and it does not discuss the management of a common DC link to improve energy efficiency of an injection molding machine.
U.S. Pat. No. 6,289,259 to Choi describes means for controlling a hydraulic actuator in an injection molding machine. A microcontroller is disposed adjacent the actuator and electrically coupled to a system control processor. The patent provides no teaching regarding a method of power balancing at the common DC link. The microcontroller simply controls the operation of the actuator in response to signals it receives from a suitably located sensor.
U.S. Pat. No. 6,275,741 to Choi describes a means for controlling the operation of an injection molding system that includes a general-purpose computer that is coupled to an operator control panel and a plurality of injection molding devices and functions. The computer is thus able to perform multi-tasking control of both the injection molding and operator control functions. The patent does not discuss power management.
U.S. Pat. No. 6,272,398 to Osborne describes a process control system for an injection molding machine that includes a first computer having a processor for controlling the molding process and a second computer having an application-specific program which allows the operator to view parameters related to the process. The patent is not concerned with power management.
U.S. Pat. No. 6,120,277 to Klaus describes an adjustable speed motor to drive either a screw in an injection molding machine or a hydraulic pump to charge an accumulator. The patent provides no teaching regarding a method of power balancing at the common DC link.
U.S. Pat. No. 6,089,849 to Bulgrin describes controls for adjustable speed motor driven pumps. The patent provides no teaching regarding a method of power balancing at the common DC link.
U.S. Pat. No. 6,034,492 to Saito describes a DC motor-generator and capacitor combination. The DC motor-generator rotates to provide electric energy to the capacitor for storage. The stored electrical energy provides an emergency power source. The patent describes a very rudimentary form of a power management system. The patent provides no teaching regarding a method of power balancing at the common DC link.
U.S. Pat. No. 5,811,037 to Ludwig describes switching off the electric drive if the length of the cooling time in the molding cycle is longer than a predetermined time. The patent provides no teaching regarding a method of power balancing at the common DC link.
U.S. Pat. No. 5,582,756 to Koyama describes the use of a DC source from a servo drive to control a heater in an injection molding machine. The patent does not discuss the use of the heater to reuse the energy regenerated during deceleration of the motor controlled by an inverter, nor managing the balance of power at a common DC link.
U.S. Pat. No. 5,580,585 to Holzschuh describes an adjustable speed motor driven pump that supplies various actuators; however, an accumulator is not shown. The patent provides no teaching regarding a method of power balancing at the common DC link.
U.S. Pat. No. 5,522,434 to Lindblom describes a very large weaving machine that is driven by a drive unit that includes a direct-current operated unit. The DC unit operates as a motor or a generator depending upon whether the drive is accelerating or decelerating. When decelerating, the electrical energy is fed back to the power supply network. The control unit for the drive unit can be a microcomputer or a personal computer unit. The patent discloses a power management system where the excess energy generated during deceleration of the drive unit is recaptured as electrical energy. This patent discloses the use of two DC motors, which can be converted as generators to recover wasted energy as electrical energy. The patent provides no teaching regarding a method of power balancing at the common DC link.
U.S. Pat. No. 5,486,106 to Hehl describes a variable capacity pump operated to maintain a constant operational pressure gradient driven by a rotary current motor connected to an adjustable frequency converter to control its speed. The patent provides no teaching regarding a method of power balancing at the common DC link.
U.S. Pat. No. 5,470,218 to Hillman describes injection blow molding apparatus that includes a plurality of injection blow molding machines each having a plurality of injection molds and blow molds. A process controller is coupled to each machine and a master processor is coupled to each of the controllers. The patent does not describe any mechanism for power management.
U.S. Pat. No. 5,362,222 to Faig describes an all-electric injection molding machine that uses vector controlled AC induction motors in its servomechanism drive systems. The vector controller means of each drive means shares a common CPU that provides pulse width modulated trigger signals, multiplexed through a switch bank in the form of either mechanical or solid state switches, to a power module of the power amplifier associated with each AC motor, one at a time. The use of either mechanical or solid state switches may prevent the simultaneous control of all servo axes in real-time. The '222 patent, however, does not provide an arrangement for retrieving nor re-using any excess power regenerated by the machine process, nor managing the balance of power at a common DC link. In addition, the vector controlled AC induction motor in '222 patent requires a sensor such as an encoder or speed sensor for detecting the angular position of the rotor of the AC induction motor. In contrast thereto, sensorless techniques having the same torque dynamics as sensored drive are well known. See, for example: J. Holtz, “Methods for speed sensorless control of AC Drives,” in Proc. IEEE IECON'93, Yokohama, Japan, 1993, pp. 415–420; A. Ferrah, K. J. Bradley, P. Hogben-Laing, M. Woolfson, G. Asher, and M. Summer, “A speed identifier for induction motor drives using real-time adaptive digital filtering,” IEEE Trans. Ind. Applicat. Vol 34, pp. 156–162, January/February 1998. Further, induction motor controlled by controller using direct torque control method, rather than vector control method, can provide the same functions and performances as disclosed in '222. See, for example: “Technical Guide No. 1—Direct Torque Control”, ABB Industry Oy, 3BFE 58056685 R0125 REV B, EN 30.6.1999.
U.S. Pat. No. 5,125,469 to Scott describes a system for storing deceleration energy from a motor vehicle and for using the stored deceleration energy to assist in accelerating the motor vehicle. The patent is useful for showing another application of a power recovery system. The patent provides no teaching regarding a method of power balancing at the common DC link.
U.S. Pat. No. 5,093,052 to Würl describes an adjustable speed drive for an electrically driven pump. The patent provides no teaching regarding a method of power balancing at the common DC link.
U.S. Pat. No. 5,052,909 to Hertzer describes a hydraulic injection molding machine that uses a variable speed motor to drive the pump. The operating signals of the variable speed motor are generated by the machine controller, in accordance with one of a plurality of stored values, calculated according to the hydraulic fluid output required of a particular phase of operation of the machine. The patent does not provide an arrangement for using accumulator means to recapture any excess power generated by the machine process, and it does not provide any means to charge the accumulator to improve energy efficiency. The patent provides no teaching regarding a method of power balancing at the common DC link.
U.S. Pat. No. 4,988,273 to Faig describes an all-electric injection molding machine that uses brushless DC motors in its servomechanism drive systems. The patent provides no teaching regarding a method of power balancing at the common DC link.
U.S. Pat. No. 4,904,913 to Jones describes a motor control system that includes a phase inverter for sensing the individual steps of the molding machine, producing a time stream of voltage levels, each of which are representative of the least amount of power required by the molding machine during its operational steps, and varying the speed of the motor in response to such voltage levels during each operation step so as to reduce the electrical power required by the machine during its cycle. The patent provides no teaching regarding a method of power balancing at the common DC link.